In imaging methods like electro(photo)graphy, magnetography, ionography, etc. a latent image is formed which is developed by attraction of so called toner particles. Afterwards the developed latent image (toner image) is transferred to a final substrate and fused to this substrate. In direct electrostatic printing (DEP) printing is performed directly from a toner delivery means on a receiving substrate by means of an electronically addressable print head structure.
Toner particles are basically polymeric particles comprising a polymeric resin as a main component and various ingredients mixed with said toner resin. Apart from colourless toners, which are used e.g. for a finishing function, the toner particles comprise at least one black and/or colouring substances, e.g., coloured pigment.
In the beginning colour electro(photo)graphy was mostly used for producing coloured images (e.g. graphic arts, presentations, coloured books, dissertations, . . . ). When the process speed of producing digital coloured images increases, other more productive applications also came into the picture (direct mailing, transactional printing, packaging, label printing, security printing, . . . ). This means that after the action of being produced by electro(photo)graphy, the toner images further have to withstand some external factors applied during the subsequent treatments such as mechanical treatments, solvent treatments and temperature treatments. The problems associated with multiple, superimposed layers of toner particles that are in one way or another fixed on a substrate are manifold, not only with respect to image quality but also with respect to image stability and with respect to mechanical stability issues.
All the above requirements can be solved by using a radiation curable toner.
The use of a transparent cover coat made out of radiation curable toner particles has been described already in e.g. U.S. Pat. No. 5,905,012 to protect an image produced by electrophotography and thereby to improve the weather resistance of an image produced by means of electrophotography.
A non image wise transparent UV curable coating has been described already in EP-A-1,288,724 to give a flexible, high gloss finishing to printed papers. Prints obtained by means of electrophotography and by the use of thermally fixable toner are thermally stable only to approximately 100° C. Packaging materials may however have to be partly heated to temperatures far above 100° C., e.g. during the production of sealed packaging. Thus for example for sealable packaging, a completely transparent, heat resistant coat layer from a toner hardening by UV light has been described in EP 1,186,961.
In EP 1,341,048 a process is described for crosslinking an unsaturated polyester under UV light.
In U.S. Pat. No. 6,461,782 a UV curable toner is described based on a cationic UV curable polymer in order to improve the mechanical resistance of the image when fusing at low temperatures.
The use of UV curable pigmented powders is already well known in the field of powder coatings (e.g. EP 792,325), but there are some major differences with respect to printer toners. The size of the particles (6-10 microns for toner versus >30 microns for powder coatings) and the particle size distribution are quite different. Also the thickness of the layers applied with powder coatings is at least a factor 3 to 4 times thicker in comparison with the printed toner images. The speed of fusing and curing is very low compared to the high speed printers which are now available in the field (e.g. Igen3, Xeikon 5000, . . . ). Powder coatings are also never applied image wise. The powders are charged by some means and brought onto the surface of the material, which has to be coated. This is all quite different from toner, which is brought either directly image wise on a substrate, or via a latent image on a photoconductor to a substrate.
In U.S. Pat. No. 5,212,526 an UV curable liquid toner has been described to improve the adhesion of the cured toner to the final substrate rather than to the surface of the image receptor during the transfuse step instead of withstanding to high temperatures. The curing here takes place during the transfer step from photoreceptor to paper.
In U.S. Patent Application Publication No. 2005/0137278 a general description is found of an emulsion aggregation (EA) toner based on styrene and an acrylate which contains also UV curable oligomers. After UV irradiation the UV curable oligomers start to crosslink and will react with the unsaturated groups of the EA monomers.
In EP 1,610,186 a process is described where toners prepared by emulsion aggregation are cured by electron beam (EB) curing. The toner contains at least a vinyl monomer and at least one EB curable polymer, and optionally a charge control additive.
In WO2005/116778 a very specific toner composition is described to be able to obtain a broad curing window independent of the colours and toner layer thickness, the particles of said toner composition comprising at least a colouring agent and a blend of radiation curable resins comprising (a) a (meth)acrylated epoxy/polyester resin and (b) a (meth)acrylated polyurethane resin, and optionally a positive or negative charge control agent. The circularity of these toner particles, and the amount of charge control agent optionally present, are not disclosed.
EP 1,096,324 describes toner particles containing at least a binder resin, a colorant, a wax component, and an external additive, wherein:    (1) the binder resin contains a component derived from butadiene, isoprene or chloroprene;    (2) said toner has a main glass transition temperature (Tg) from 40° C. to 70° C. as measured by differential scanning calorimetry (DSC);    (3) the toner satisfies a specific relationship between its specific surface area (BET method) at 23° C. and 65% relative humidity and its specific surface area (BET method) at 50° C. and 3% relative humidity;    (4) as measured with a flow type particle image analyser, the toner particles have an average diameter from 2 to 10 μm, an average circularity from 0.950 to 0.995 and a circularity standard deviation less than 0.04; and    (5) as measured by gel permeation chromatography (GPC), the toner has a main-peak molecular weight from 2,000 to 100,000 and contains from 5% to 60% by weight THF-insoluble matter.The external additive present in the toner particles of EP 1,096,324 may an inorganic fine powder (such as silica, titanium or alumina) optionally treated with a silicone, or it may include a lubricant, an abrasive, an anti-caking agent, a conductivity-providing agent, or a developing performance improver such as white or black fine powder with a polarity reverse to that of toner particles. The toner of EP 1,096,324 may also optionally comprise a charge control agent in an amount from 0.1% to 10% by weight of the binder resin.
The circularity feature of the toner particles of EP 1,096,324 is described as a combination with the other features of said toner, in particular the chemical nature of the monomer(s) present as component(s) of the binder resin. EP 1,096,324 teaches that since diene monomers include no oxygen atom, there is no site which may absorb water in air, so that any leak of electric charges may hardly occur in the toner. Moreover since diene monomers have two radically polymerizable double bonds and can easily have a three-dimensional structure, they can contribute to increasing viscosity and formation of a network structure, therefore improving the distribution of pigments dispersed in the toner particles, and improving tints of toners better than monomers such as styrene, vinylcyclohexane, or divinylbenzene. As a consequence, a diene monomer is an essential component of the binder resin of EP 1,096,324. However it should be noticed that EP 1,096,324 fails to mention any type of curing or submitting the toner composition to any type of radiation.
In the Journal of Imaging Science and Technology (2002) 46:313-320 an academic study by Nash et al. is presented on the charging properties of toners and carriers. A comparison in made between toners with internal mixed charging agents and external mixed charging agents in their charging behaviour on a CCA (Charge Control Agent) and non-CCA coated carrier. This study teaches that the place where the CCA is located (e.g. inside toner, outside toner, on the carrier surface) determines very much the charging performance and also charging value (positive or negative) and that a lot of care is needed when CCA's are mounted. No guidance is given on the effect of CCA's inside toner systems on properties of the toner or toner image. This reference does not teach the circularity of the toner particles tested, or the nature of the resin matrix present in the toner particles tested.
In a lot of the above applications where UV curable can be used a very wide range of substrates are used, e.g. paper, foils and laminates with various thicknesses. It is not obvious to obtain and realize a good transfer efficiency and an acceptable print quality on the different substrates, which have all their specific electrical and surface properties.
By the fact that the printing speed of the current digital presses is increasing and can be adjusted according to the application and or type of substrate, more and higher demands with respect to toner developabilty and chargebiltiy are required. Also the fact that in digital colour printing the page content can be different for each colour and from job to job, places higher demands for developabilty and chargebiltiy on the toner.
From all those references only a general description of radiation curable toner is found and a high quality performing radiation curable toner is still not attainable with the above teachings. In particular it is known to the skilled person that the charge stability and charge build up of rounded toner particles with a circularity of at least 0.95 is significantly worse than that of non-rounded toners. Therefore there is still a need in the art, which is one problem addressed by the present invention, for dry toner particles exhibiting both a circularity of at least 0.95 and a suitable combination of charge stability and charge build up for high performance printing.